[willamette-fcst] Silverton Hills AM Forecast _doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT TUES JUL 23, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from 2:00 p.m. to 4:00 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



A typical summertime pattern continues over the Pacific NW with a flat upper level ridge over the state today.  Morning low clouds are confined to the north end of the valley this morning.  Pressure gradients are currently onshore with a 2.2 mb onshore gradient from Newport to Salem and a 1.0 mb onshore gradient from Salem to Redmond.



Expect little change from yesterday with the ridge maintaining strength this afternoon and evening.  Transport and surface winds will remain northerly through the day.  Sunny skies and warm temperatures will dominate.



(Salem Airport data for Monday, July 22nd: High 87°F; Rainfall .00")

(Maximum Ventilation Index expected today:  40)

TODAY'S FORECAST:



Expect sunny skies with above average temperatures continuing.



Salem's high temperature today will be near 90.

Relative humidity will drop to 45-50% by noon.  Minimum RH 30-35%.

Surface winds: N to NE at 5-10 mph.

Transport winds: N at 10-15 mph.

Maximum mixing height:  3500 ft.

Sunset tonight: 8:48 p.m.



EXTENDED DISCUSSION:



The upper level ridge begins to move to the east on Wednesday.  This will allow some minor trough energy to develop and move into the area from the south, along with some mid-level moisture, and it has the potential to bring some scattered thunderstorms over the Cascades late Wednesday.  Low level wind flow will remain mostly northerly.



On Thursday the upper ridge will likely begin to rebuild over the region.  Pressure gradients will be mostly weak leaving the region under persistent northerly flow. This scenario now appears likely to persist through Friday.



The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text



Notes:



     1.  Mixing height, as used here, is the lowest height at which the

         potential temperature exceeds the potential temperature at the

         surface.  As a practical matter it is the approximate height to

         which a smoke plume will rise assuming good ignition, dry fuels, and

         winds less than about 15mph.



     2.  Transport winds are a layer average through the mixing height,

         weighted slightly toward the winds at the top of the layer.



     3.  Ventilation Index is the height of the mixing layer times

         the transport wind speed divided by 1000.



     4.  Surface wind direction is the general expected wind direction.

         At a specific point surface winds are highly dependent on local

         terrain conditions.



This forecast is provided under an agreement between the Oregon Department of

Agriculture and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.



Doug Wesley

ODF Meteorologist


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